Not only has strong demand for tablet computers — led by Apple’s (AAPL) iPad — cut into notebook PC sales, but it has started to be felt in the market for high-definition televisions.

That’s the thrust of a note issued Wednesday by Hudson Square Research’s Daniel Ernst. The evidence:

• His weekly survey shows the average price per square inch of HDTVs down 23.4% year over year so far this month vs. the 18% decline he recorded in June 2010.
• He’s been checking the stores and observed “thin activity” in the TV aisles, compared with “strong activity” for tablets and smartphones.
• Total worldwide TV unit sales were up only 0.9% in the first quarter of 2011, according to Display Search, vs. 17.4% in 2010. For HDTVs, first-quarter unit sales were up 8.9% year over year vs. 30.9% in 2010.

Ernst acknowledges that the lack of activity in the TV aisles could be due in part to the time of year, and that the disaster in Japan and weak housing trends in the U.S. could also be factors.

But it makes sense. A new gadget is a new gadget. And if you can watch The Daily Show or True Grit or Wimbledon in high-definition on a $500 tablet perched on your belly, you may not feel as compelled to spend $3,000 for a 55-inch TV set in your living room.